Marine seismic exploration is traditionally conducted by firing an acoustic source which generates a collapsing air bubble. The collapse of the bubble generates acoustic pulses that propagate through the water and into the earth. By analyzing the reflected seismic wave field detected by the receiver(s) during the survey, the geophysical data pertaining to reflected signals may be acquired and these signals may be used to form an image indicating the composition of the Earth near the survey location. The seismic wave field within the water column can transmit pressure waves (P-waves) while a wave field on the ocean bottom can transmit both P-waves and shear waves (S-waves), in addition to more complex wave fields.
Two basic methods are used to record these wave fields. Within the water column and near the surface a network of hydrophones can be towed behind a seismic vessel, in proximity of a sound source. Towed array seismic data acquisition systems can be models of productivity, and the density and network of sensors can be configured to collect vast amounts of seismic data while towing at speeds up to six knots. Towed array seismic surveys can also be significantly less expensive than competing technologies such as ocean bottom cables and nodal systems. The data quality, however, may be somewhat less relative to “stationary” acquisition methods, as acoustic noise is generated when the streamers are towed. The lack of S-wave detection can also be a limitation on imaging capability.
Cable based ocean bottom seismic systems such as those disclosed in U.S. Pat. Nos. 6,775,203 and 8,446,797, each of which is incorporated by reference herein, exemplify some embodiments of cable based ocean bottom systems. These seismic acquisition systems serially deploy dual sensor packages having hydrophones and particle motion sensors such as geophones or accelerometers to the ocean floor. Power and telemetry are supplied via electrical and optical conductors with sensor stations serially and coaxially connected at periodic intervals of 25 to 50 meters.
Ocean bottom seismic acquisition system(s) available from ION Geophysical Corporation of Houston, Tex. are capable of operating in up to 2,000 meters of water. In order to reliably deploy and recover the system the electrical conductors are overwound with a steel armor package. The armor package, typically steel, can be deployed accurately and rapidly. Maximum deployment speed is limited by the free fall velocity of the system, determined by the difference in specific gravity of the system and seawater. Armor based systems may take up to 30 minutes to descend 2,000 meters. Consequently the surface deployment vessel speed is dependent on the system free fall decent time. As steel is significantly heavier than sea water there is a maximum depth achievable relative to the tensile strength of the armor package. Steel wire systems are depth limited as the addition of more steel simply increases tensile loads. Armor based cables offer fast and accurate positioning but suffer reliability issues due to the complexity of the power and telemetry requirements.
Nodal systems are autonomous seismic recorders which are deployed and recovered by remotely operated vehicles (ROV) or by a carrier line. Each node has a battery and recording system eliminating the requirement for a heavy power and telemetry system. Instead light weight synthetic ropes are used to deploy and retrieve nodal systems. The depth of the seismic survey is now limited to the node survivability and deployment techniques as the density of the carrier line is similar to sea water. The impact on deployment rate and positioning accuracy is a significant shortcoming of using a light weight carrier line as the carrier line specific gravity is close to sea water. Deployment speed is significantly impacted as it may take up to 1 hour for the system to descend to 2,000 m.
Near the surface cross currents significantly impact touch down accuracy in deep water. Currents near the ocean bottom however are usually insignificant. Retrieval of the near neutrally buoyant system is advantageous during the recovery operation as winching loads are significantly less than armor based ocean bottom systems. Needed is a deployment solution that transitions near surface high currents while rapidly deploying nodes in deep water.